Venturi Cowls

Venturi cowl encourage the smoke to travel vertically, giving it a better chance of being dispersed. A concentric shroud, venturi cowl or parallel rain excluder will allow the smoke to travel upwards and significantly reduce the amount of rain entering the chimney.
Because rain usually falls at an angle, the rain hits the inside sides of the shroud, travels down the sides and out the bottom without entering the chimney.

Anti Down Draft Cowls

The anti down draft cowls help to stop downdraft caused by the air blown back down the chimney. The downdraft will result in puff of smoke coming out the stove or chimney from time to time. Down draft can also happen during windy days. Note that if the smoke is continuously coming out of the appliance it is not a downdraft.